1. Field of the Invention
The invention concerns an intruder detection module, particularly one adapted to be mounted in the wall of a home or business, the module incorporating an improved and simplified system for detecting and signalling the presence of an intruder within the areas viewed by the system.
2. Prior Art
Numerous systems have been designed and are presently in use to sense and signal the presence of an intruder within a defined area.
Some intrusion detection systems are active in nature, producing a signal within or confined to a secure area, the signal being effected by the presence of an intruder and this effect being employed to actuate an alarm or other signal. Not only do such active systems require a continuous energy input, but they are prone to take alarms and their presence can be detected by the intruder, all of which cause the system to be ignored, avoided or otherwise negated.
Some intrusion detection systems are of a passive nature. Most passive systems detect the presence of an intruder in a defined area by sensing and responding to visible or non-invisible radiation of the intruder. Rossin U.S. Pat. No. 3,839,640, dated Oct. 1, 1974, presents an example of an intruder detection system responsive to infrared radiation, a system that is remarkably free from fake alarms.
Intrusion detection systems are subjected to a number of non-intruder related inputs, all of which must be distinguished from an intruder related input if the system is to prove practical in use. Inputs which can cause passive infrared systems to prodduce false alarms include reflected sunlight from an airplane or automobile, or lights being directed onto the detector from automobile headlights, or a flashlight out of the detection area. An intermittent electric heater, or a flickering light bulb in the detector's field of view, also can actuate present systems, causing a false alarm.
An intrusion detection system must avoid false alarms, particularly from such environmental occurrences. Certain communities, for example, levy a fine of hundreds of dollars for each false alarm to which its police force must respond. While achieving an intruder detection system that avoids such false alarms is therefore quite important, it is of course equally important to detect the presence of an intruder, even when the intruder is taking steps to avoid detection, such as by wearing white clothing while crossing a white background or while crossing a monitored background only after having somehow adjusted his external temperature to closely approximate that of the background temperature. In short, detecting the presence of an intruder while not producing numerous false alarms presents a significant problem, one which has only been partially solved by previous intruder detection systems.
It is also important to provide an intruder detection system of reasonable size, preferably one which requires no external power source and is small enough to mount in a reasonably inconspicuous fashion at any of various locations throughout a building. For example, a system that could be incorporated in a module that can be mounted in a wall, and that approximates in shape and style an ordinary wall switch or outlet face plate, would be quite desirable because it would be relatively inconspicuous in appearance.
Among other desirable features of an intruder detection system are features to automatically detect a malfunction of the system. As has been noted, it is as important to detect an intruder as to avoid false alarms from extraneous environmental occurrences. The system must be in a proper operative condition if it is to achieve these results. Accordingly, a system which incorporates certain self-checking and self-testing features would be highly desirable.